1. Field of the Invention
This invention relates to a plastic ovenware.
2. Description of the Prior Art
Ovenwares, namely, cooking vessels used in electronic ovens and subjected to a combination of microwave irradiation and electric heating therein have various requirements such as excellent heat resistance, mechanical strengths, electrical characteristics, resistance to staining by foods and fine appearance. The heat resistance, in particular, needs to be very high because not only high frequency but also electric heating are applied. For instance, ovenwares need to cause no deformation even at 200.degree. to 250.degree. C. and to have practical mechanical strengths.
Wholly aromatic polyesters have various excellent properties owing to their molecular structure and are outstanding among all resins particularly in heat resistance. Of the wholly aromatic polyesters, those produced from terephthalic acid, isophthalic acid, p-hydroxybenzoic acid, or its derivatives, and 4,4'-dihydroxydiphenyl or its derivatives are injection-moldable and excellent in heat resistance, chemicals resistance, radiation resistance, insulation, etc. and accordingly are in extensive use in many fields including electricity and electronics.
Further, the wholly aromatic polyesters, although they have a large dielectric loss tangent of 0.004 to 0.02 similarly to conventional polyesters, have a high crystallinity and a very high heat distortion temperature of 293.degree. C.; therefore, even when a microwave has been applied to them, they stay below the heat distortion temperature and do not deform easily.
Furtheremore, the wholly aromatic polyesters have improved heat resistance, improved rigidity and decreased mold shrinkage when they are blended with a fibrous reinforcing material such as a glass fiber, a carbon fiber or the like or with an inorganic filler material of powder, needle or flake form such as calcium carbonate, magnesium carbonate, aluminum hydroxide, glass beads, titanium oxide, mica, clay, a silica powder, talc, molybdenum disulfide, graphite, wollastonite, a potassium titanate fiber or the like.
However, the wholly aromatic polyesters have various drawbacks. In injection molding, they have a high molding temperature; a slight difference of molding temperature greatly changes the melt viscosity; and they are orientated severely. Moreover, they solidify very quickly in a mold. Therefore, the molded article is high in anisotropy, has non-uniform flow marks in appearance, is poor in surface smoothness and does not look fine. When a food containing an edible oil, vinegar and a sauce is placed in this molded article and in such a state the molded article is subjected to repeated use as an ovenware, staining of the inner surface becomes increasingly severe; an unpleasant odor stays inside; and the non-tackiness of the inner surface deteriorates leading to poor food releasability.
Meanwhile, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 36154/1984, the molded article of wholly aromatic polyester can be remarkably improved in anisotropy and appearance by incorporating talc and titanium oxide of rutile structure into the wholly aromatic polyester. However, the resulting molded article has many fine voids on the surface due to insufficient flow in a mold and this causes detrioration in food releasability when the molded article is subjected to long term use as an ovenware, as seen in a molded article made only from a wholly aromatic polyester. This problem exists also when other fibrous reinforcing material or inorganic filler material is incorporated into a wholly aromatic polyester.